1. Field of the Invention
The present invention relates to a control apparatus for an automatic transmission.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2001-165301 (hereinafter, referred to as the document '301) suggests a technique provided in a control device for an automatic transmission of a parallel shaft type. The automatic transmission includes a plurality of gears and a plurality of hydraulic clutches supported by an input shaft and an output shaft arranged in parallel to one another, and changes the speed by supplying fluid pressures to the hydraulic clutches. The control device accurately calculates a friction coefficient of the hydraulic clutch and properly converts a clutch torque into a fluid pressure value.
More specifically, the technique described in the document '301 calculates a rotation difference of the hydraulic clutch, calculates a state value (Sommerfeld number), calculates a clutch friction coefficient μ, and converts a clutch torque into a fluid pressure value by using the calculated clutch friction coefficient μ.
In an automatic transmission, if a heat load is continuously input to a hydraulic clutch, for example, because speed change is continuously performed, the hydraulic clutch is excessively heated, and the friction coefficient of the hydraulic clutch is decreased. In the technique of related art, the plate temperature of the hydraulic clutch is not considered, and hence, the fluid pressure supplied to the hydraulic clutch may be insufficient. A similar phenomenon may occur in an automatic transmission using a planetary gear.